This invention relates generally to temperature control systems and in particular to a thermostat for use therein and an anticipator for a thermostat.
In the past, various apparatus, such as anticipators or the like, and various methods have been employed in various types of thermostats for controlling the cycle rate thereof with accompanying acceptable droop. Droop is a condition well known in the art occurring in all thermostats, and droop may be thought of as the difference in the average temperature controlled by the thermostat when minimum heat is required as compared with the average temperature when maximum heat is required. When an anticipator is employed with the temperature sensing means of the thermostat, droop is the difference between the basic temperature differential of the thermostat and the absolute temperature rise of the temperature sensing means of the thermostat due to anticipator heat. For instance, a good thermostat for contemporary heating systems may have generally about 5 cycles to about 7 cycles per hour with a minimum or acceptable droop of less than approximately 2.degree.F.
In general, thermostats are optimized to function with a given set of characteristics, i.e., cycle rate and droop. These characteristics depend on the basic temperature differential of the thermostat, i.e., the difference between the "on" temperature and the "off" temperature of the temperature sensing means of the thermostat, the thermal mass of the temperature sensing means, the heating of the temperature sensing means, the heating of the temperature sensing means by an anticipator, and the cooling of the temperature sensing means. Since the temperature differential of the thermostat, the thermal mass of the temperature sensing means and the cooling thereof are fixed values for any one thermostat, it is apparent that the thermostat's characteristics are fixed except for the influence of the anticipator thereof. Therefore, the characteristics of any one thermostat will change when the heat of the anticipator is changed, i.e., an increase in heat will increase both the cycle rate and the droop. As well known in the art, when the droop condition for any one thermostat is bettered, a relatively slower cycle rate is effected. However, a slow cycle rate tends to cause large swings, i.e., large operating differentials, in room temperature. It is therefore apparent that a system for maintaining generally constant anticipator heating is desired.
In the operation of any particular thermostat, it is desirable to maintain or control a desired cycle rate for the thermostat with acceptable droop, and this has been accomplished in the past by providing various types of anticipators or auxiliary heaters in conjunction with the thermostat. In general, at least some of the past anticipators were disposed within thermostats for transmitting heat to the temperature sensing mechanism or bimetal thereof. Of course the added heat caused the thermostat to become satisfied before the temperature of the space controlled by the thermostat reached the selected or set-point temperature thereof. In this manner, satisfaction of the thermostat was anticipated to prematurely turn off the heating system for the controlled space thereby to eliminate or at least control overshoot of the temperature in the controlled space.
In at least some of the past thermostats, the switching or other power responsive elements thereof created heat which was also additive to that sensed by the temperature sensing mechanism of the thermostat thereby to affect the droop and cycle rate characteristics of the thermostat. In some instances, an attempt was made to physically separate or isolate these power responsive, heat creating elements from the temperature sensing mechanism or bimetal of the thermostat. At least one of the disadvantageous or undesirable features of this particular type of past thermostat is believed to be the impractability of effecting such separation of parts within the thermostat as well as the increased size and cost thereof which may have been engendered by such separation.
In other past thermostats, an auxiliary heater or anticipator and a regulating control therefor were incorporated to differentially provide supplemental heat additive to that of the aforementioned power responsive, heat creating elements of the thermostat. In this manner, the heat added to the temperature sensing mechanism of the thermostat was maintained at a constant value thereby to effect a selected and constant droop throughout the operating range of the thermostat. At least some of the disadvantageous or undesirable features of this particular type of past thermostat is believed to be that the anticipator and regulating control therefor was continuously on thereby to consume and waste power, and with the addition of the regulating control for the anticipator, the circuitry of the thermostat naturally became more complex, the thermostat may have increased in size, and of course, the cost thereof undoubtedly increased.
In some other past thermostats, anticipators of the solid state type, such as thermistors for instance, were utilized in rather complicated bridge circuitry, and it may be that such complexity itself is a disadvantageous or undesirable feature not to mention the cost and maintenance which may have been involved.
Solid state elements, such as the aforementioned thermistors, have also been utilized as heaters in other circuitry and systems. One such system employed a thermistor as a heater for controlling expansion and contraction of a thermally responsive material for driving a piston or control arm for instance. However, it is apparent that such usage of thermistors is not analogous to the anticipator art for thermostats.
Some of the past anticipators utilized in thermostats were generally constituted by a wound coil with a movable arm for adding or subtracting a selected number of coil turns into the coil energizing circuit. In this manner, the amount of heat added by the anticipator to the temperature sensing mechanism of the thermostat could be regulated. However, one of the disadvantageous or undesirable features of this particular type of anticipator was that it is believed to be necessary to adjust it in the field during installation of the thermostat. Another disadvantageous or undesirable feature of such adjustable anticipators is believed to be that the field adjustment thereof may not have been correlative with the power draw of the control unit, such as a solenoid, relay or gas valve for controlling the system heat source with which the thermostat and anticipator was in circuit; also, incorrect cycle rates would be incorrect for desired confort level due to the improper amount of heat from the anticipator being transferred to the sensing element.
Further, in many of the past anticipators, heat was transferred primarily by conduction from the heat generating means thereof to the means for transferring such heat to the bimetal of the thermostat. One of the disadvantageous features of this particular type of past anticipator is believed to be that the heat generating means necessarily was operated at relatively low temperatures and was responsive to voltage variations so that the rate of heat generated was variable.